For example, when acoustic equipment such as a speaker is installed in a hall or a gymnasium to radiate a loud sound wave from the speaker, music or voice from the speaker is sometimes difficult to listen to because of the presence of a resonant frequency in this space (loud sound space in which the acoustic equipment is installed). To be specific, if the loud sound wave from the speaker contains a component of the resonant frequency in large amount, resonance occurs in a frequency of this component in the loud sound space. A resonant sound is like “won . . . ” or “fan . . . . ” The resonant sound is not a sound wave to be radiated from the speaker and makes it difficult to listen to the music or the voice from the speaker.
To avoid this, the resonant frequency in the loud sound space is detected, and a dip filter or the like is provided at a forward stage of the speaker in the acoustic equipment to remove the component of the resonant frequency. Thereby, resonance is unlikely to occur in this loud sound space, making it easy to listen to the music or the voice from the speaker.
In order to enable the dip filter to give such effects, it is necessary to determine a frequency characteristic of the dip filter so that the resonant frequency in this loud sound space is a frequency to be removed.
Traditionally, an operator or a measuring person for the acoustic equipment distinguished the loud sound from the speaker or the resonant sound depending on their senses of hearing to make judgment of the resonant frequency, and the resonant frequency was set in the dip filter as the frequency to be removed. And, a damping level (depth) or quality factor (Q) of the dip filter was set so as to prevent resonance.
Even if the operator or the measuring person can distinguish the resonant frequency, it is not easy to set the frequency characteristic of the dip filter. In particular, it is not easy to appropriately set the damping level (depth) or the quality factor (Q) of the dip. The damping level of the dip is maximized (depth is maximized) and the quality factor is minimized (Q is minimized) if priority is given to only prevention of resonance. But, if the damping level becomes too high or the quality factor becomes too low, a sound quality of the acoustic equipment may be degraded, or music or voice may be difficult to listen.
Some skill or experience is required to set the damping level or the quality factor of the dip appropriately in order to avoid occurrence the above mentioned event. These factors (damping level or the quality factor of the dip) are not accurately set if the setting depends on the skill or experience. Furthermore, this has impeded automatic measurement and automatic adjustment of the acoustic equipment installed in the loud sound space or the like.